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Equipment
Radiotherapy
Introduction
Radiotherapy has two distinct practices: external beam radiotherapy (EBRT) and brachytherapy; depending on whether the ionizing radiation source is external to the patient or internal/in close proximity to the patient, respectively. Separate types of medical equipment are utilized for the practice of EBRT and brachytherapy. Most external beam radiotherapy is carried out with photon or electron beams; lately other particles such as protons, and carbon ions are also being utilized. In brachytherapy, radiation sources are placed directly into the target volume (intracavitary or interstitial brachytherapy) or on to a target (surface mould or intraoperative radiotherapy).
External beam equipment
External beam radiotherapy began with superficial and orthovoltage therapy using x-ray tubes and teletherapy containing sealed radioactive sources. Megavoltage therapy matured in the 1950’s with the development of the Cobalt-60 machines along with the medical linear accelerator. Cyclic particle accelerators, such as the betatron and the microtron, have also found their use in radiotherapy. Proton and light-ion therapy facilities were also developed, and their number all around the world is constantly increasing.
Cobalt-60 units
Treatment devices incorporating gamma-ray emitting sources for use in external beam radiotherapy are called radionuclide teletherapy units. These units use radioactive isotopes emit high-energy gamma rays. Two gamma-emitting radionuclides have been widely used for teletherapy: Cobalt-60 and Cesium-137. The use of Cesium-137 for external beam radiotherapy was discontinued during the 1980s due to its low specific activity (large source size and short treatment distance). Cobalt-60 is in turn the most widely used isotope for teletherapy as it offers a good compromise between the energy of emitted photons, half-life and specific activity. The source movement from beam-on to beam-off (storage) position is done with mechanical or pneumatic methods. Modern radionuclide teletherapy units can be equipped with advanced software control, rotating gantry, collimation, in-room imaging, and a treatment couch.
The advantages of the cobalt-60 teletherapy unit over the linac are the simplicity of design and less infrastructure services needed. This translates in general to lower running and maintenance costs, which can be important considerations for some sites. The disadvantages are the source decay and the additional radiation safety and security considerations needed for the high-activity radioactive cobalt-60 source.
Linacs
Clinical linear accelerators (linacs) allow for photon energies higher than those from cobalt-60 machines and have become the most widely used radiation source in radiotherapy. Linacs have a design which is compact and efficient, and provide electron or x-ray therapy with megavoltage beam energies. In addition, linacs offer excellent versatility for use in radiotherapy through isocentric mounting. Various types of linacs are available for clinical use: Some provide x-rays only in the low megavoltage range (e.g., 4 or 6 MV), while others provide both x-rays and electrons at various megavoltage energies. A typical modern high energy linac can provide more than one photon energy (e.g., 6, 10 and 15 or 18 MV) and several electron energies (e.g., 6, 9, 12, 15, 18 and 22 MeV).
In a linac, electrons are produced by thermionic emission (emission of electrons from a hot cathode into a vacuum tube) in the electron gun and then are accelerated in special evacuated structures called accelerating waveguides. X-ray photons are then produced when the accelerated electrons collide with a metallic target (a thin layer of a high-Z material such as tungsten). Linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine: gantry, gantry stand or support, modulator cabinet, patient support assembly (e.g., treatment table), control console. In recent years, a range of distinct “unconventional” designs for linacs have appeared on the market. Such as machines with a linac mounted on a robotic arm, machines with magnetic resonance imaging (MRI) incorporated into the unit, and machines with a miniature linac waveguide mounted on a CT type gantry.
Although modern linacs are versatile and allow for treatment of multiple treatment sites, they require the facility to have access to many infrastructure services. This can include many of the following: continuous supply of chilled water, compressed air supply, an uninterruptable power supply, power conditioner, and room air and humidity control. The requirements are dependent on the vendor and should be considered early in the planning process. These additional requirements add to the complexity and cost of the machine in comparison to a cobalt unit.
kV teletherapy units
Kilovoltage x-ray units are often intended as an alternative to electron beams from linacs. The application of low-energy superficial x-ray units can be extended to deeper treatments with the utilization of orthovoltage x-ray units. Superficial and orthovoltage x-rays used in radiotherapy are produced with x-ray tubes. The main components of a radiotherapeutic x-ray machine are: an x-ray tube, a ceiling or floor mount for the x-ray tube, a target cooling system, a control console, and an x-ray power generator. The beam quality for these units is specified in terms of half-value layer (HVL) expressed in thickness (in mm) of aluminium or copper for superficial or orthovoltage x-ray beams, respectively.
Superficial X-ray units generally operate in the range of 50–150 kVp. Short source to skin distances are implemented, in the range of 5–15 cm, to maximize the dose fall-off with depth. The radiation field is defined by applicators of various sizes and customized shapes can be created with lead cut-outs. The unit comes with various aluminium filter sets, which with different kV settings create radiation beams of different qualitites. Beam-on duration is usually controlled with a timer system.
The combined superficial/orthovoltage unit typically operates in the range of 50 kV–300 kV, with applicators at short SSDs for superficial therapy and larger applicators for use at SSDs of 25 cm–50 cm for higher energies from 100 kV–300 kV. These units come with various aluminium and copper filter sets, which with different kV settings create radiation beams of different quality. Beam-on duration is usually controlled by an internal monitor chamber that must be calibrated separately for each beam quality.
Clinically, the dose for superficial x-ray beams is most often prescribed at the skin surface, while for treatments with orthovoltage x-ray beams a point at the centre of the target volume is chosen, generally located at depths ranging from a few mm to a few cm. Absolute dose determinations are therefore performed either in air, in combination with backscatter factors, or in a phantom using appropriate depth dose data. Relative dose distributions are generally based on tables published in literature or on isodose curves provided by the manufacturer of x-ray machines.
Particle accelerators
Proton therapy (PT) is an external beam radiotherapy modality that uses protons rather than photons. In the last decade, there has been a significant growth of new PT facilities due to encouraging results achieved and technological developments (e.g., patient positioning and intensity modulated treatment delivery modalities) that have made the dose delivery with PT more accurate than in the past. According to the Particle Therapy Co-Operative Group data, 92 PT facilities were in operation worldwide in April 2020 (https://www.ptcog.ch/index.php/facilities-in-operation-restricted). By the end of 2018, more than 190,000 patients have been treated with PT (PTCOG - Patient Statistics).
The particle accelerator is a machine that accelerates ions to a high energy beam using electromagnetic fields. Currently, clinically available particles are protons and carbon ions. According to a joint ICRU/IAEA recommendation, they are both light ions, i.e., ions with atomic number equal or smaller than that of Neon (Z<=10). The main advantages of ion particle therapy are the concentration of the dose to the tumour and the sparing of healthy tissues in the body. The ion radiotherapy facilities make use of: cyclotrons, cyclo-synchrotrons and synchrotrons.
Cyclotrons are compact accelerators producing beams with a fixed extraction energy. The desired maximum range of ions is usually 30 cm in soft tissue, requiring 220 MeV of kinetic energy. Cyclotrons are normally used to produce proton beams. The ability to generate ion beams heavier than protons with these accelerators is still under development. Synchrotrons, on the other hand, allow protons or ions heavier than protons to be produced in ion beams with variable energy. Two types of proton beam delivery modalities are currently used in clinics: One is the broad-beam system using scattered or uniformly scanned beams, while the second one is the pencil-beam system (PBS) or spot scanning system which uses intensity-modulated scanned beams. In the broad-beam delivery systems, the beam is spread uniformly and then conformed to the target by customized collimators and range compensators. In the PBS system, a narrow beam is electromagnetically scanned over the target volume in a sequence specifically designed for each target, allowing intensity modulate proton therapy (IMPT). The dose is delivered to the patient by a large number of small segments that consist of a sequence of mono-energetic pencil beams, where most of the energy deposition is then concentrated only at a well-defined location near the end of the range (called spots).
Brachytherapy
Brachytherapy involves the process of delivering a radiation dose to the cancer patient with sealed radioactive sources placed into, or in close proximity to, the tumour. Over the years, various sealed radioactive sources have been used in various configurations to treat a range of cancer sites, including skin, eye, prostate, breast, cervix, lung, nasopharynx, and soft tissue tumours. According to the dose rate to the point of dose prescription, brachytherapy is classified into three categories: Low Dose Rate (LDR) brachytherapy (0.4 to 2 Gy/h), Medium Dose Rate (MDR) brachytherapy (2 Gy/h to 12 Gy/h) and High Dose Rate (HDR) brachytherapy(>12 Gy/h). Pulsed Dose Rate Brachytherapy (PDR) delivers the dose in a large number of small fractions with short intervals, mimicking the radiobiology of LDR brachytherapy.
Currently, brachytherapy is mainly performed with automatic afterloading equipment that uses radioactive sources capable of delivering HDR treatments. The versatility of HDR brachytherapy has enabled this treatment to be offered on an outpatient basis, with two to four fractions being typically delivered over a period of one or two weeks. Iridium-192 is the most used radioisotope for HDR brachytherapy. Its half-life of 74 days requires a frequent source change, every 3 to 4 months. Recently, machines using a miniaturized Cobalt-60 source are also available with a half-life of 5.26 years, requiring fewer source exchanges over the lifecycle of the afterloader. Dosimetric differences between Cobalt-60 and Iridium-192-based brachytherapy are minimal. However, the higher energy of the Cobalt-60 radiation (1.25 MeV versus 0.38 MeV average photon energy) requires thicker shielding to ensure radiation protection.
Treatment planning systems
Treatment planning is the process of creating a treatment plan for the patient based on the radiation oncologist’s dose prescription. Modern treatment planning is generally computerized rather than manual and is achieved with a treatment planning system (TPS). This system is a multifaceted software application that receives patient images, allows delineation of the tumour, target volumes and organs at risk from the patient images, creates treatment plans with different levels of user intervention, allows 3D dose distribution visualization and plan optimization, includes treatment plan review features such as dose statistics and dose-volume-histograms. The treatment planning system (TPS) is the combination of software and hardware used to generate the treatment beam geometry and calculate the expected dose distribution in the patient’s tissue. Many of software configurations and hardware are possible, making the TPS highly configurable equipment.
Phase-space database for external beam radiotherapy
The Monte-Carlo (MC) method has become a widely accepted tool in recent years for computational simulation of clinical components of linear accelerators, especially beam generation and collimation, and for the interaction of radiation beams with patients and dosimetric materials. Advanced simulation algorithms and sophisticated computer codes based on the most recent set of cross-section data have become tools for radiotherapy treatment planning of patients at research centres. Accurate simulations of detector response to accelerator-produced beams are being conducted today for dosimetric purposes.
The accuracy of the calculations is dependent on accurate characterization of the radiation source, meaning the detailed description of the energy, direction, position and type of all primary and secondary particles emerging from the clinical accelerator. This description is known as the phase-space (phsp) data of the accelerator. To generate such data, very detailed information must be obtained from accelerator manufacturers, sometimes at the level of blueprints. This poses a severe limitation on who can have access to phsp data to develop or implement new radiotherapy techniques. Furthermore, the simulation of phsp requires extensive Monte-Carlo expertise for execution and verification.
The IAEA has established a public database of phsp data for clinical accelerators and Cobalt-60 units used for radiotherapy applications. Such a database provides a harmonized set of data common to different applications and gives scientists access to phsp data. The data are freely available to medical physicists and dosimetry research teams in Member States at the following link https://www-nds.iaea.org/phsp/phsp.htmlx
Equipment for emerging technology
Image guided radiotherapy
Imaging has always been a part of radiotherapy, however, in the last 10–15 years the advent of digital imaging and computer control has seen the introduction of more imaging options in radiotherapy, both in simulation and at the time of treatment with dedicated in-room imaging devices. Image guided radiotherapy (IGRT) systems are based on direct integration of a kilovoltage or megavoltage imaging system on the linac. IGRT can be performed using many systems including cone beam computed tomography (CBCT), a CT-on rails, a megavoltage computed tomography (MVCT), or on-line imaging using paired planar imagers.
IGRT is an important component in the safety and effectiveness of advanced techniques, such as intensity modulated radiotherapy (IMRT). The implementation of IGRT is a stepwise process requiring establishment of IGRT implementation committee, as well as specification, procurement, installation, acceptance testing, and commissioning of IGRT equipment. After the IGRT implementation committee has been appointed, it will assess the clinical needs and determine the priorities for the implementation of IGRT. Specifications need to be developed for the required equipment. In general, there are two scenarios: (i) the purchase of a whole new treatment unit with IGRT or (ii) the upgrading of an existing treatment unit to have IGRT capability. It is important to allow sufficient time for physics staff training before the equipment arrives so that trained staff is in place to carry out acceptance testing and commissioning. Commissioning the new technology for the intended clinical application within the department does not only depend on the actual IGRT equipment used, but also on the intended use and all other equipment (hardware and software) the IGRT tools interface with. The imaging equipment for IGRT requires similar quality assurance as conventional diagnostic radiology equipment (i.e., image quality, geometrical accuracy and dosimetry), plus additional quality assurance for image guidance (i.e., positioning, isocenter coincidence).
Stereotactic radiation therapy (SRT)
Improvements in tumor localization, imaging and dosimetry in radiotherapy has allowed for the implementation of hypofractionated radiotherapy for disease sites where conventional fractionation (around 2 Gy per fraction) was previously the norm or where curative radiotherapy was not attempted. SRT comprises high-precision irradiation techniques that use multiple, photon radiation beams to deliver a high dose of radiation. Stereotactic localization, using frame-based or frameless techniques, is used to treat lesions. Regarding dose fractionation, SRT is divided into stereotactic radiosurgery (SRS), in which the total dose is delivered in a single treatment session, and stereotactic radiotherapy, in which the total dose is delivered in multiple fractions (usually less than 8 treatments). Originally this technique was only used when treating lesions located in the brain, however now a number of extra-cranial malignancies are treated using stereotactic localization (stereotactic body radiotherapy (SBRT)).
SRT can be delivered using a wide variety of treatment devices and may incorporate specialized dose delivery methods such as IMRT. Requirements for SRT comprise secure patient immobilization, accurate tumour localization, and a solution for respiratory motion, if relevant. Dedicated SRT equipment is now commercially available and includes units that incorporate an array of separate Cobalt-60 sources producing collimated beams directed to a specific focal point, linacs that combine IGRT with modern immobilization and respiratory motion technology, and units that incorporate a miniature linac mounted on a robotic arm. All steps involved in SRT must be verified experimentally to ensure reliable and accurate performance of the hardware and software (i.e., end-to-end tests). Conventional linacs are also capable of delivering SRT when commissioned for this purpose. The most important requirement of a linac used for SRT is the mechanical stability and accuracy of its isocentre. In SRT, dose conformity to a relatively small target volume is extremely important, and the target dose homogeneity requirement is often relaxed to allow for optimization of the target dose conformity.
Conventional and CT simulators
Imaging for treatment planning involves X-ray imaging of the patient in the intended treatment position and with the corresponding patient immobilization accessories.
The conventional simulator is intended for radiotherapy treatment planning for treatments on either a linac or cobalt-60 teletherapy unit. The simulator can also be used for brachytherapy treatment planning. The conventional simulator includes gantry, collimator, X-ray tube and X-ray detector, patient couch and control console. In the conventional simulator, an X-ray tube replaces the high-energy radiation source of the treatment unit, allowing planar X-ray imaging of the intended treatment ports or orthogonal imaging for isocentre verification. The conventional simulator facilitates the practice of 2D radiotherapy and offline verification and markup of patients for 3D-CRT.
Currently, most departments utilize a dedicated computed tomography (CT) scanner, termed a CT simulator. The CT simulator takes a diagnostic CT scanner and adds a flat top couch and external lasers. Ideally the dedicated CT scanners will be equipped with a larger bore compared to the standard bore of a diagnostic CT scanner. A larger bore is needed in radiotherapy treatment simulation because the patient in the treatment position may be tilted or have arms extended (using an immobilization device such as a breast board) or be placed laterally offset on the couch. A carbon fibre flat couch top is required identical to that of the linac or cobalt-60 units in the radiotherapy department.
The acquired patient images allow the delineation of the treatment volume, the tumour and the nearby organs at risk and are also used to provide reference images to compare against treatment verification images. The acquired 3D patient geometry and tissue mass density information allows accurate calculation of radiation absorbed dose in 3D to the patient. For patients where the tumour position within the patient is affected by internal motion (e.g., lung tumour varying during the phases of the breathing cycle) it is advantageous to acquire CT data sets at different stages of the breathing cycle or to synchronize image acquisition with the breathing cycle.